The present invention generally relates to imaging and image-guided navigation. In particular, the present invention relates to a system and method for improved calibration of equipment used in imaging and image-guided operations.
Medical practitioners, such as doctors, surgeons, and other medical professionals, often rely upon technology when performing a medical procedure, such as image-guided surgery or examination. A tracking system may provide positioning information for the medical instrument with respect to the patient or a reference coordinate system, for example. A medical practitioner may refer to the tracking system to ascertain the position of the medical instrument when the instrument is not within the practitioner's line of sight. A tracking system may also aid in pre-surgical planning.
The tracking or navigation system allows the medical practitioner to visualize the patient's anatomy and track the position and orientation of the instrument. The medical practitioner may use the tracking system to determine when the instrument is positioned in a desired location. The medical practitioner may locate and operate on a desired or injured area while avoiding other structures. Increased precision in locating medical instruments within a patient may provide for a less invasive medical procedure by facilitating improved control over smaller instruments having less impact on the patient. Improved control and precision with smaller, more refined instruments may also reduce risks associated with more invasive procedures such as open surgery.
Tracking systems may be ultrasound, inertial position, or electromagnetic tracking systems, for example. Electromagnetic tracking systems may employ coils as receivers and transmitters. Typically, an electromagnetic tracking system is configured in an industry-standard coil architecture (ISCA). ISCA uses three colocated orthogonal quasi-dipole transmitter coils and three colocated quasi-dipole receiver coils. Other systems may use three large, non-dipole, non-colocated transmitter coils with three colocated quasi-dipole receiver coils. Another tracking system architecture uses an array of six or more transmitter coils spread out in space and one or more quasi-dipole receiver coils. Alternatively, a single quasi-dipole transmitter coil may be used with an array of six or more receivers spread out in space.
The ISCA tracker architecture uses a three-axis dipole coil transmitter and a three-axis dipole coil receiver. Each three-axis transmitter or receiver is built so that the three coils exhibit the same effective area, are oriented orthogonally to one another, and are centered at the same point. If the coils are small enough compared to a distance between the transmitter and receiver, then the coil may exhibit dipole behavior. Magnetic fields generated by the trio of transmitter coils may be detected by the trio of receiver coils. Using three approximately concentrically positioned transmitter coils and three approximately concentrically positioned receiver coils, for example, nine parameter measurements may be obtained. From the nine parameter measurements and one known position or orientation parameter, a position and orientation calculation may determine position and orientation information for each of the transmitter coils with respect to the receiver coil trio with three degrees of freedom.
In medical and surgical imaging, such as intraoperative or perioperative imaging, images are formed of a region of a patient's body. The images are used to aid in an ongoing procedure with a surgical tool or instrument applied to the patient and tracked in relation to a reference coordinate system formed from the images. Image-guided surgery is of a special utility in surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac procedures, interventional radiology and biopsies in which x-ray images may be taken to display, correct the position of, or otherwise navigate a tool or instrument involved in the procedure.
Several areas of surgery involve very precise planning and control for placement of an elongated probe or other article in tissue or bone that is internal or difficult to view directly. In particular, for brain surgery, stereotactic frames that define an entry point, probe angle and probe depth are used to access a site in the brain, generally in conjunction with previously compiled three-dimensional diagnostic images, such as MRI, PET or CT scan images, which provide accurate tissue images. For placement of pedicle screws in the spine, where visual and fluoroscopic imaging directions may not capture an axial view to center a profile of an insertion path in bone, such systems have also been useful.
When used with existing CT, PET or MRI image sets, previously recorded diagnostic image sets define a three dimensional rectilinear coordinate system, either by virtue of their precision scan formation or by the spatial mathematics of their reconstruction algorithms. However, it may be desirable to correlate the available fluoroscopic views and anatomical features visible from the surface or in fluoroscopic images with features in the 3-D diagnostic images and with external coordinates of tools being employed. Correlation is often done by providing implanted fiducials and adding externally visible or trackable markers that may be imaged. Using a keyboard or mouse, fiducials may be identified in the various images. Thus, common sets of coordinate registration points may be identified in the different images. The common sets of coordinate registration points may also be trackable in an automated way by an external coordinate measurement device, such as a suitably programmed off-the-shelf optical tracking assembly. Instead of imageable fiducials, which may for example be imaged in both fluoroscopic and MRI or CT images, such systems may also operate to a large extent with simple optical tracking of the surgical tool and may employ an initialization protocol wherein a surgeon touches or points at a number of bony prominences or other recognizable anatomic features in order to define external coordinates in relation to a patient anatomy and to initiate software tracking of the anatomic features.
Generally, image-guided surgery systems operate with an image display which is positioned in a surgeon's field of view and which displays a few panels such as a selected MRI image and several x-ray or fluoroscopic views taken from different angles. Three-dimensional diagnostic images typically have a spatial resolution that is both rectilinear and accurate to within a very small tolerance, such as to within one millimeter or less. By contrast, fluoroscopic views may be distorted. The fluoroscopic views are shadowgraphic in that they represent the density of all tissue through which the conical x-ray beam has passed. In tool navigation systems, the display visible to the surgeon may show an image of a surgical tool, biopsy instrument, pedicle screw, probe or other device projected onto a fluoroscopic image, so that the surgeon may visualize the orientation of the surgical instrument in relation to the imaged patient anatomy. An appropriate reconstructed CT or MRI image, which may correspond to the tracked coordinates of the probe tip, may also be displayed.
Among the systems which have been proposed for effecting such displays, many rely on closely tracking the position and orientation of the surgical instrument in external coordinates. The various sets of coordinates may be defined by robotic mechanical links and encoders, or more usually, are defined by a fixed patient support, two or more receivers such as video cameras which may be fixed to the support, and a plurality of signaling elements attached to a guide or frame on the surgical instrument that enable the position and orientation of the tool with respect to the patient support and camera frame to be automatically determined by triangulation, so that various transformations between respective coordinates may be computed. Three-dimensional tracking systems employing two video cameras and a plurality of emitters or other position signaling elements have long been commercially available and are readily adapted to such operating room systems. Similar systems may also determine external position coordinates using commercially available acoustic ranging systems in which three or more acoustic emitters are actuated and their sounds detected at plural receivers to determine their relative distances from the detecting assemblies, and thus define by simple triangulation the position and orientation of the frames or supports on which the emitters are mounted. When tracked fiducials appear in the diagnostic images, it is possible to define a transformation between operating room coordinates and the coordinates of the image.
In general, the feasibility or utility of a system of this type depends on a number of factors such as cost, accuracy, dependability, ease of use, speed of operation and the like. Intraoperative x-ray images taken by C-arm fluoroscopes alone have both a high degree of distortion and a low degree of repeatability, due largely to deformations of the basic source and camera assembly, and to intrinsic variability of positioning and image distortion properties of the camera. In an intraoperative sterile field, such devices are typically draped, which may impair optical or acoustic signal paths of the signal elements they employ to track the patient, tool or camera.
More recently, a number of systems have been proposed in which the accuracy of the 3-D diagnostic data image sets is exploited to enhance accuracy of operating room images, by matching these 3-D images to patterns appearing in intraoperative fluoroscope images. These systems may use tracking and matching edge profiles of bones, morphologically deforming one image onto another to determine a coordinate transform, or other correlation process. The procedure of correlating the lesser quality and non-planar fluoroscopic images with planes in the 3-D image data sets may be time-consuming. In techniques that use fiducials or added markers, a surgeon may follow a lengthy initialization protocol or a slow and computationally intensive procedure to identify and correlate markers between various sets of images. All of these factors have affected the speed and utility of intraoperative image guidance or navigation systems.
Correlation of patient anatomy or intraoperative fluoroscopic images with precompiled 3-D diagnostic image data sets may also be complicated by intervening movement of the imaged structures, particularly soft tissue structures, between the times of original imaging and the intraoperative procedure. Thus, transformations between three or more coordinate systems for two sets of images and the physical coordinates in the operating room may involve a large number of registration points to provide an effective correlation. For spinal tracking to position pedicle screws, the tracking assembly may be initialized on ten or more points on a single vertebra to achieve suitable accuracy. In cases where a growing tumor or evolving condition actually changes the tissue dimension or position between imaging sessions, further confounding factors may appear.
When the purpose of image guided tracking is to define an operation on a rigid or bony structure near the surface, as is the case in placing pedicle screws in the spine, the registration may alternatively be effected without ongoing reference to tracking images, by using a computer modeling procedure in which a tool tip is touched to and initialized on each of several bony prominences to establish their coordinates and disposition, after which movement of the spine as a whole is modeled by optically initially registering and then tracking the tool in relation to the position of those prominences, while mechanically modeling a virtual representation of the spine with a tracking element or frame attached to the spine. Such a procedure dispenses with the time-consuming and computationally intensive correlation of different image sets from different sources, and, by substituting optical tracking of points, may eliminate or reduce the number of x-ray exposures used to effectively determine the tool position in relation to the patient anatomy with the reasonable degree of precision.
However, each of the foregoing approaches, correlating high quality image data sets with more distorted shadowgraphic projection images and using tracking data to show tool position, or fixing a finite set of points on a dynamic anatomical model on which extrinsically detected tool coordinates are superimposed, results in a process whereby machine calculations produce either a synthetic image or select an existing data base diagnostic plane to guide the surgeon in relation to current tool position. While various jigs and proprietary subassemblies have been devised to make each individual coordinate sensing or image handling system easier to use or reasonably reliable, the field remains unnecessarily complex. Not only do systems often use correlation of diverse sets of images and extensive point-by-point initialization of the operating, tracking and image space coordinates or features, but systems are subject to constraints due to the proprietary restrictions of diverse hardware manufacturers, the physical limitations imposed by tracking systems and the complex programming task of interfacing with many different image sources in addition to determining their scale, orientation, and relationship to other images and coordinates of the system.
Several proposals have been made that fluoroscope images be corrected to enhance their accuracy. This is a complex undertaking, since the nature of the fluoroscope's 3D to 2D projective imaging results in loss of a great deal of information in each shot, so the reverse transformation is highly underdetermined. Changes in imaging parameters due to camera and source position and orientation that occur with each shot further complicate the problem. This area has been addressed to some extent by one manufacturer which has provided a more rigid and isocentric C-arm structure. The added positional precision of that imaging system offers the prospect that, by taking a large set of fluoroscopic shots of an immobilized patient composed under determined conditions, one may be able to undertake some form of planar image reconstruction. However, this appears to be computationally very expensive, and the current state of the art suggests that while it may be possible to produce corrected fluoroscopic image data sets with somewhat less costly equipment than that used for conventional CT imaging, intra-operative fluoroscopic image guidance will continue to involve access to MRI, PET or CT data sets, and to rely on extensive surgical input and set-up for tracking systems that allow position or image correlations to be performed.
Thus, it remains highly desirable to utilize simple, low-dose and low cost fluoroscope images for surgical guidance, yet also to achieve enhanced accuracy for critical tool positioning.
Magnetic fields may affect x-rays and other image energy sources. Additionally, gravity may affect geometry of an x-ray system. Focal length and piercing point of x-rays may change depending upon the position of a C-arm or other mobile component of an imaging system. A difference between an imaging angle and an angle of the Earth's magnetic field may cause distortion that affects a resulting image. Additionally, an operator or patient may bump the C-arm or other component of an imaging system during operation or positioning, which may affect a resulting image. Thus, there is a need for improved calibration to reduce an effect of distortion on an image.
Registration is a process of correlating two coordinate systems, such as a patient image coordinate system and an electromagnetic tracking coordinate system. Several methods may be employed to register coordinates in imaging applications. “Known” or predefined objects are located in an image. A known object includes a sensor used by a tracking system. Once the sensor is located in the image, the sensor enables registration of the two coordinate systems.
U.S. Pat. No. 5,829,444 by Ferre et al., issued on Nov. 3, 1998, refers to a method of tracking and registration using a headset, for example. A patient wears a headset including radiopaque markers when scan images are recorded. Based on a predefined reference unit structure, the reference unit may then automatically locate portions of the reference unit on the scanned images, thereby identifying an orientation of the reference unit with respect to the scanned images. A field generator may be associated with the reference unit to generate a position characteristic field in an area. When a relative position of a field generator with respect to the reference unit is determined, the registration unit may then generate an appropriate mapping function. Tracked surfaces may then be located with respect to the stored images.
However, registration using a reference unit located on the patient and away from the fluoroscope camera introduces inaccuracies into coordinate registration due to distance between the reference unit and the fluoroscope. Additionally, the reference unit located on the patient is typically small or else the unit may interfere with image scanning A smaller reference unit may produce less accurate positional measurements, and thus impact registration.
Typically, a reference frame used by a navigation system is registered to an anatomy prior to surgical navigation. Registration of the reference frame impacts accuracy of a navigated tool in relation to a displayed fluoroscopic image. Therefore, a system and method that improve registration of the reference frame would be highly desirable. Improved registration may help to decrease error between reference frames and improve navigated tracking accuracy.
Aspects of imaging system variability may be addressed using tracking elements in conjunction with a calibration fixture or correction assembly to provide fluoroscopic images of enhanced accuracy for tool navigation and workstation display. The calibration fixture and use of the calibration fixture in tracking are described in further detail in U.S. Pat. No. 6,484,049 by Seeley et al., issued on Nov. 19, 2002, and U.S. Pat. No. 6,490,475 by Seeley et al., issued on Dec. 3, 2002. A reference unit may also be used, as described in further detail in U.S. Pat. No. 5,829,444 by Ferre et al., issued on Nov. 3, 1998. Radiopaque calibration markers, such as ball bearings (BBs), are used to calibrate components in an imaging system.
Calibration fixtures or reference units may be used to reduce registration error for a registration or reference frame and improve accuracy in navigated tracking of an instrument. A reference frame may include a calibration fixture. The calibration fixture may be removably attached in a precise position with respect to the camera or to the patient. One or more tracking elements or markers may be included in the calibration fixture. A tracking element may be a point-origin defining tracking element that identifies spatial coordinates and/or orientation of the tracking element and, therefore, an object to which the tracking element is attached. Thus, a tracking element may with one or more measurements determine a position of markers in the calibration fixture and a position and orientation of the fixture itself or a surface to which the fixture is attached.
Current fixtures use very radiopaque, discrete markers in radiolucent material. Thus, a need exists for improved calibration fixtures and calibration markers. Typically, an array of discrete, dark markers, such as ball bearings, is arranged in multiple planes for use in calibration. A calibration system watches for spikes and attenuations in a recorded curve to identify the ball bearings in the image.
Examples of calibration fixtures are described in U.S. Pat. No. 5,829,444, mentioned above, and a U.S. patent application entitled “Method and System for Improved Correction of Registration Error in a Fluoroscopic Image”, by Douglas Johnson and Lewis Levine, filed on Jun. 2, 2004 (Ser. No. 10/859,767), which is herein incorporated by reference. FIG. 1 illustrates an example of a calibration fixture 50 that may be used in improved coordinate frame registration and tracking accuracy. The fixture 50 may include one or more marker plates or sheets 52 of radiolucent material, such as an acrylic (e.g., Lexan) or polymer plate. Each sheet holds an array of radiopaque point-like markers 54, such as stainless steel balls (e.g., ball bearings or BBs). The one or more plates holding the BBs may be affixed at or near to the face of the camera imaging assembly so as to allow accurate calibration of the entire volume of interest while occupying a sufficiently small space that the camera may be positioned closely to the patient. The illustrated calibration fixture 50 includes a releasable clamp assembly 51, with a clamp handle 51a, or other attachment device configured to attach directly on or over the face of the camera assembly. Additionally, the calibration fixture 50 may include an attachment point for a tracking sensor. The structure of the calibration fixture 50 is predefined and modeled to serve as a reference. That is, radiopaque markers, sensors, and/or other structures in the calibration fixture 50 are located and measured during manufacturing or prior to use in imaging. Characterization data from the structure of the calibration fixture 50 is used to register the image and navigation coordinate systems. For example, characterization data describing reference markers in the fixture 50 is loaded into a tracking system prior to scanning. Tight manufacturing tolerances and/or modeling are used to model the calibration fixture 50 as a reliable reference to measure and correct registration error.
FIG. 2 illustrates another example of a calibration fixture 60 that may be used in improved coordinate frame registration and tracking accuracy. The calibration fixture 60 may be affixed at or near an object being imaged, such as a patient. The calibration fixture 60 includes an attachment point 61 for a tracking sensor. The calibration fixture 60 includes an array of radiopaque calibration markers, such as BBs. In an embodiment, a size of the second calibration fixture 60 is minimized to reduce an impact of the fixture 60 on a resulting image. Holes or openings, for example, may also be left in the calibration fixture 60, as shown in FIG. 4, to reduce its profile and impact on imaging. The structure of the second calibration fixture 60 is predefined and modeled to serve as a reference. That is, radiopaque markers, sensors, and/or other structures in the calibration fixture 60 are located and measured during manufacturing or prior to use in imaging. Characterization data from the structure of the calibration fixture 60 is used to register the image and navigation coordinate systems. For example, characterization data describing reference markers in the fixture 60 is loaded into a tracking system prior to scanning. Tight manufacturing tolerances and/or modeling are used to model the calibration fixture 50 as a reliable reference to measure and correct registration error.
The BBs or other radiopaque or calibration markers may be of different sizes in the different planes or may be of the same size. In an embodiment, the BBs are of the same size, e.g., about one or two millimeters in diameter. The BBs or other markers appear in an image and are discernable from anatomy or other interesting objects.
In an embodiment, a radiopaque object, such as a metal or other material object, for example a BB, may be placed into holes on a radiolucent marker sheet such that all marker coordinates are known. Alternatively, marker plates may be manufactured by circuit board microlithography techniques to provide desired patterns of radiopaque markers, for example as metallization patterns, on one or more thin radiolucent films or sheets. The calibration fixtures 50, 60 may also be fabricated as a single block of a suitable radiolucent material with holes drilled to accommodate BBs or other markers. BBs or other markers may be arranged at multiple levels and multiple positions in the fixture.
One suitable radiolucent material is a structural foam of the type used in aircraft wings for lightweight structural rigidity. The radiolucent material may also be employed in separate thin marker-holding sheets. In an embodiment, the selected polymer or foam, and the number and size of the markers, are configured to remain directly in the imaging beam of the fluoroscope device and be imaged in each shot, while the position of the fixture is tracked. The fixture materials are selected to avoid introducing any significant level of x-ray absorption or x-ray scattering by the plates, sheets or block, and the size and number of markers are similarly chosen to avoid excessive shadowing of the overall image, while maintaining a sufficiently dense image level for their detectability, so that both the imaging source radiation level and the resulting image density scale remain comparable to currently desired operating levels. In an embodiment, BBs or other markers are arranged in a pattern at one or more levels, with a different pattern at each level. Further, when more than one array at different depths is used, the patterns may be positioned so that as the source/camera alignment changes, BBs of one pattern cast shadows substantially distinct from those of the other pattern(s). Thus, calibration fixtures 50, 60 are predefined structures known with respect to a tracker.
However, use of BBs or other calibration markers in a fixture may impose distortion or artifacts in resulting images. Although some work has been done to remove such artifacts, some distortion still remains, and there is a need for an improved system and method for reducing artifacts introduced in an image by calibration markers. A system and method for improved calibration and distortion reduction would be highly desirable.
Generally, a goal of intrinsic geometry calibration is to determine a location of an x-ray focal spot in relation to an x-ray detector. On existing x-ray systems, such as fixed-room or mobile C-arms, the focal spot location may vary by 10 mm or more over the full range of motion of the C-arm structure. A source of this variation may be elastic deflection of the C-arm itself, bearing backlash, and other component motions. Knowing the precise location of the focal spot is important for 3D reconstruction and 2D navigation.
Fluoroscopy-based 3D imaging and 2D and/or 3D surgical navigation require accurate characterization of imaging parameters such as the camera focal length, piercing point, and optical distortion parameters, etc. Since C-arm devices are mobile imaging equipment, camera calibration is usually performed with every X-ray exposure to compensate for the mechanical deflection of C-arm for different clinical setups. Calibration is typically accomplished by deploying a calibration fixture between the X-ray detector and source that encloses an array of discrete, radiopaque markers such as ball bearings (BBs) arranged in multiple planes. The physical presence of the radiopaque BBs produces shadows on the acquired fluoro-image for estimation of the camera parameters is undesirable for image quality.
Depending on the size and location of the BBs, possible consequences of introducing BBs to the imaging chain include loss of important anatomical features (e.g., 2D cardiovascular imaging), introduction of metal scattering artifacts (e.g., 3D imaging), and bad pixel identification (e.g., flat panel detector IQ).
As mentioned above, prior geometry calibration procedures use a calibration phantom, which typically is comprised of a number of discrete fiducials arranged in a three-dimensional pattern. One such phantom uses a series of BBs arranged in a helix around an x-ray transparent cylinder. In an offline calibration procedure, images of the phantom are acquired throughout the motion trajectory of the C-arm and the intrinsic geometry parameters are computed. These parameters are assumed to remain unchanged and are used for subsequent in vivo scans. Another method uses one or more planes of BBs or crosshairs affixed to the detector surface. This calibration phantom is used clinically. After an image of the anatomy is taken, the intrinsic parameters are calculated and the image artifacts from the fiducials are removed via image processing techniques.
Both of the methods described suffer from disadvantages. The helical phantom and offline procedure assumes that the parameters will remain unchanged. Wear and damage to the device may affect the accuracy of the stored parameters. Furthermore, there may be situations where the user unknowingly is flexing the C-arm by unintended collision with the operating table. The second method suffers from image degradation from the removal of the image artifacts. Also, the depth of the calibration phantom (e.g., 8-10 cm) compromises the usable patient volume between the x-ray source and detector.
Thus, systems and methods that provide intrinsic parameter calculation for a variety of images would be highly desirable. Systems and methods that minimize image degradation would also be highly desirable. Additionally, systems and methods that provide imaging system calibration or characterization without the introduction of markers would be highly desirable.